1. Field of the Invention
The present invention relates to a resistor calibration system, and more particularly, to a resistor calibration system capable of calibrating an on-chip termination resistor.
2. Description of the Prior Art
On-chip termination (OCT) is the technology where the termination resistor for impedance matching in transmission lines is located inside a semiconductor chip instead of on a printed circuit board (PCB). In order for a transmission line to minimize distortion of the signal, the impedance of every location on the transmission line should be uniform throughout its length. If there is any place in the line where the impedance is not uniform for some reason (e.g., open circuit, impedance discontinuity, different materials), the signal gets modified by reflection at the impedance change point which results in distortion, ringing and so forth. In order to avoid impedance mismatch, termination impedance with the equivalent amount of impedance should be placed at the end point of the transmission line. This is described as “termination”. There are several ways of termination depending on how the resistors are connected to the transmission line. Parallel termination and series termination are examples of termination methodologies.
Instead of having the necessary resistive termination located on the motherboard, the termination located inside the semiconductor chips is called OCT. In order to achieve better signal integrity and higher operational speed of data buses, the accurate OCT values are necessary. The conventional OCT calibration implementation is accomplished with an external reference resistor, which is connected to the chip via a pad. The external reference resistor has an accurate resistance value. This reference resistor is connected to a target resistor in the chip via the pad, to adjust the resistance value of the target resistor by monitoring a voltage on the pad, where the voltage is obtained by voltage division of the resistance values of the external reference resistor and the target resistor. The voltage obtained by the voltage division may further be compared with a reference voltage via a comparator, in order to control the calibration of the target resistor.
However, the comparator may usually possess an offset error between its two input terminals. For example, the output of an ideal comparator may change state when the voltages on the two input terminals of the comparator become equal. When an input offset error occurs in the comparator, the output of the comparator may change state when the voltage on an input terminal is slightly higher or lower than the voltage on another input terminal. This results in an error of the OCT calibration. Thus, there is a need for improvement over the prior art.